1. Field of the Invention
The invention relates to a method for processing a suspension of solid particles in a liquid, such as the effluent of a crystallizer (hereinafter also to be called: crystal slurry) or solid particles suspended in a liquid extraction medium. The invention also relates to apparatus for processing such a suspension.
2. Description of the Related Art
Such a method and apparatus are known from WO-A-98/27240. According to this publication, a suspension consisting of molten metal having solid metal particles therein (unmelted or crystallized) is passed through a washing column in order to separate the relatively pure solid particles from the contaminated liquid fraction (hereinafter also to be called: mother liquor). The washing column contains one or more filters, each of which is included in a separate pipe, which extends in the longitudinal direction through the interior of the washing column, while the suspension is supplied at one extremity of the column and moves along the pipes in the direction of the other extremity of the column. The mother liquor passes the filters and thus ends up in the pipes to be discharged. As via the filters, mother liquor is extracted from the suspension, a porous bed of particles is formed in the column. At the other extremity of the column, means are present for disintegrating or breaking up the bed of particles formed. The removed solid (crystalline) material is melted and a portion of this melt is fed to the particle bed as washing liquid, in order to wash the particles in their own melt. The means for disintegrating or breaking up the crystal bed formed consist of a rotating knife.
A drawback of this known method and apparatus is that the use of a rotating knife, in practice, can entail problems. Melted metals, such as aluminum, are not only hot (the melting point of aluminum is approximately 650° C.) but, moreover, are generally chemically highly reactive (aggressive), in particular with respect to metal objects. For this reason, all parts which are in contact with the liquid metal are to be designed in a material which is resistant to this contact, for instance in ceramics or carbon. This holds in particular for the rotating knife and parts such as bearings. In addition, the rotating knife is to be driven by a motor which is disposed outside the column. The rotating movement is to be transmitted via a shaft which is guided through the column wall. This can entail additional problems with respect to the sealing used which, naturally, must stop the melted material in an adequate manner, without this hindering the rotation of the shaft. All this leads to complicated constructions and to process control which is potentially sensitive to malfunction, for instance in that the rotating knife may become wedged. For processing suspensions based on molten metals, materials such as ceramics or materials based on carbon are prerequisite. Processing these materials into the desired form (for instance into a rotating knife) is a costly matter. Further, the rotating parts require regular maintenance, to which end the process control needs to be halted. With columns of larger diameters, shutting down the apparatus and repair can be a costly matter.